Semiconductors have found wide use in a range of optical, electrical, and opto-electronic devices including amplifiers, detectors, and emitters. For the majority of these devices, high crystalline quality must be insured to prevent unwanted traps and scattering mechanisms. Typically these devices require the epitaxy of particular thicknesses of various semiconductors on one another. Initially, epitaxy was used to form bulk materials with different n and p doping in various regions. The advantage of introducing a different semiconductor, or other material, was realized and devices began to use heterojunctions to localize the electronic states and tailor their precise energy. A heterojunction occurs when dissimilar materials are brought together. This heterojunction is important in semiconductor devices, since each material has a different band gap and band alignment. By appropriate selection of these materials, electrons or holes may be localized in particular regions or block particular types of transport. A heterojunction is the junction, that is, the site, where the two dissimilar materials are brought together; a heterostructure is the overall layer structure utilizing the junctions. For example, a quantum well is a heterostructure which is formed through the use of two heterojunctions. Quantum wells are used to optimize device performance and produce new structures such as resonant tunneling diodes. More recently, researchers have started using layers with reduced dimensionality. In going from the bulk semiconductor to the quantum well, the electronic states were localized in one direction, however still remained free in the other two perpendicular directions. In the case of a quantum wire, the electronic states are confined in two directions and remain free in the other. Finally, for a quantum dot, the electronic state (electrons and holes) is confined in all directions. The confinement produces a wavefunction which has a limited spatial extent in other directions: the wave function is periodic like in the bulk.
In addition to the localization of the wave function, a change in the density of states, that is, the number of electrons at a particular energy, occurs. For a bulk material, the density of state is proportional to the square root of the energy, E½. As the confinement increases from a quantum well to a quantum wire to a quantum dot, the density of states are a constant, E−1/2, and finally a delta function. The formation of quantum dots having tailorable localized electronic states with delta function electronic states gives rise to many unique properties, which have the potential to lead to new devices. Some of these advantages are entangled states, ultra sharp wavelength detection and emission, enhanced non-linear electro-optic effects, and reduced temperature dependence of within band transitions, just to name a few. Currently quantum dots are finding uses in lasers and detectors, are being considered for single electron devices, and have the potential to be a qubit for quantum computing.
Quantum dots may be formed spontaneously, under certain conditions, during lattice-mismatched epitaxial growth. In these growths, the dots are formed upon an existing structure. Typically, the final layer of this pre-existing structure is a flat layer and is nearly dislocation free. The layer used to form self-assembled quantum dots (SAQDs) must be strained relative to the average in-plane lattice constant of the final layer of the underlying structure. Every crystal has a particular lattice constant, i.e. the size of the repeating structure in the material. Lattice mismatched materials have different “repeat” sizes and hence, in order for the structures to form without breaks in the crystal structure (i.e., grow epitaxially), one or both of the layers must be strained in order accommodate this mismatch. Low strain systems are typically around 1%. Highly strained systems typically refer to a system with ˜7% or greater strain, however, three-dimensional growth is seen at times with around 3% strain. The magnitude of the strain here is |aepi-asub|/asub in which it is assumed all the strain is incorporated in the epitaxial layer. The symbol | | refers to the absolute value. The lattice constants of the epi-layer and the substrate are aepi and asub respectively.
Deposition of this layer proceeds by two different growth modes. FIG. 1 shows a diagram of a typical self-assembled quantum dot formed during epitaxial growth. FIG. 1 shows a pre-existing structure at 100, a final layer of underlying structure at 101, and an original SAQD material at 102, which additional may form a wetting layer. Three-dimensional growth occurs during the deposition of the wetting layer and forms a quantum dot depicted at 105. In FIG. 1, the dot 105 is shown after growth of a layer 103 to cap the dot. In order for the quantum dot 105 to be an actual quantum dot, the electron or hole must be localized in the quantum dot or surrounding region. Finally, a capping layer 103 that is nearly lattice matched to the substrate is deposited. The purpose of the capping layer 103 is to complete the confining potential of the dots. This process allows the formation of SAQDs in quantum wells or buried SAQDs. In FIG. 1, the final layer of the underlying structure has been broken from the underlying, pre-existing structure at 100 for clarity.
A wetting layer formation in layer 102 in FIG. 1 may or may not be present, depending on which growth mode is active. It is referred to as a wetting layer since no contact angle exists between the materials. When this occurs the material is said to wet the surface. If growth initially proceeds in a flat manner and then converts to rough, three-dimensional growth, the process is termed Stranski-Krastanov and a wetting layer is expected. However, if the growth is immediately is rough and three-dimensional, the process is termed Volmer-Weber and no wetting layer is expected. In both of these processes, the quantum dots form in an effort to minimize the strain in the epitaxial layer.
In each case of self-assembled quantum dots in the prior art, the dot itself is composed of a single material. The purpose of these self-assembled quantum dots is to induce the confinement of the electron and holes spatially in three directions. Many examples of these types of growth are found in the molecular beam epitaxy of one III-V or II-VI semiconductors onto another III-V or II-VI material. A few specific examples are InAs on GaAs, InP on GaAs, GaSb on GaAs, InAs on InP, In0.5Ga0.5As on GaAs. One of the major limitations of this type of quantum dot structure is that the confined region is composed of a single material. This limitation restricts the tuning of the electron and hole wavefunctions and their associated properties. In addition, the electrons and holes are localized in a single region. The present invention solves the limitations associated with having the confined regions composed of a single layer. With a quantum dot composed of more than one layer, as disclosed by the method of the present invention, benefits include tunability of separate electron and hole states while maintaining close proximity and hence overlap between the electron and hole wavefunctions. Also by the introduction of multiple layers, many additional configurations of the dots may be realized.